The present invention relates to a method and apparatus for intensifying an alkaline pulping process according to which at least soap-containing spent liquors from digestion and soap-containing washing filtrates from brown stock washing are recirculated in the digester house, the brown stock washing plant or between these. The invention relates in particular to treatment of at least the soap fraction separated from liquors and filtrates in order to separate fiber material therefrom.
In a chemical alkaline pulping process, lignocellulose-containing material, such as wood chips, is treated by digesting it in a solution the active chemicals of which are mainly sodium sulphide and sodium hydroxide, or sodium hydroxide alone. After digestion, spent liquor is separated from the pulp which is carried to further treatment such as washing, screening and bleaching. Chemicals are recovered from the spent liquor, i.e. black liquor, for production of fresh cooking solution.
During the digestion process, greases and resin acids of wood saponify and form sodium soaps, sulphate soap. Conventionally, soap is separated from black liquor prior to feeding the liquor to evaporation and combustion to form green liquor and further white liquor. As soap is lighter than liquor it rises to the surface and is readily decanted off. It is important to remove soap as fully as possible because it is a strong scummy substance and thus causes many problems in the process. However, it should be noted that part of the soap has been dissolved into the black liquor and thus it is not separated to the surface of the liquor.
In addition to the digestion, soap is transferred from the pulp to the liquor also during washing of the pulp. The solubility of soap is different from that of other dry solids and thus it may not be as easily washed as other dry solids. In filter washing, soap is removed from the pulp at a later stage than other dry solid. Foaming soap also hampers filter washing of pulp.
As known, chemical pulp may be produced by an alkaline cooking process both in batch digesters and in continuous digesters. Instead of always feeding the black liquor from the digester to recovery of chemicals, hot black liquor may be used to pretreat wood chips before the digestion stage itself in order to improve the process. In a continuous digestion, hot black liquor may be recirculated to pretreat wood chips in the impregnation stage as has been described for example in EP patent no. 527 294. Hot, pressurized black liquor may also be flashed to produce steam and to raise the dry solids content of the black liquor. In batch digestion, black liquor may be stored at different temperatures in liquor tanks, "liquor batteries", for use in a pretreatment of chips.
Recirculation of black liquor provides several advantages. For example, the heat economy of the process is improved and, when sulphur is present in the digestion, black liquor augments the volume of sulphur compounds in the cellulose pulp which in the impregnation stage have a very favourable influence of decreasing the Kappa number of the pulp. Recirculation of black liquor brings about problems, also, as the soap contained in the black liquor accumulates in the process if it is not efficiently removed from the solutions recirculated.
Efficient removal of soap in the processes described above has not been paid necessary attention to before. EP patent application no. 520 452 discloses a method having three tanks for black liquor. At the beginning of displacement with washing filtrate, which takes place after digestion, black liquor is introduced into a first hot liquor battery essentially at the cooking temperature and pressure and at the dry solids content reached during the digestion. Black liquor, the temperature of which corresponds to the boiling temperature of liquor in an atmospheric pressure, is fed into a second black liquor battery. The dry solids content of the black liquor in the second battery is lower than that of the black liquor in the first battery because the black liquor mentioned first contains remarkable amounts of wash liquid and consequently also its soap content is higher. According to this EP publication, the second black liquor is used to heat the white liquor to be introduced into the digester and after that the black liquor is transferred to a tank in which soap is separated from it. Subsequently, the black liquor having a temperature of less than 100.degree. C. is used to impregnate the chips of a new digestion batch. The chips are impregnated after that also with the hot black liquor from the first tank.
It has been noticed recently, however, that the measures of removing soap described above are not adequate. If soap is not removed efficiently enough from liquids reused, such as black liquor and washing liquids, soap will accumulate in the pulp in the digester and will dissolve from it during further treatment. When pulp is further removed from the digester to a blow tank and a brown stock washing, soap causing strong froth formation gives rise to problems in the blow tank and wash filters and thus causes production disturbances and decreases the pulp quality.
As described above, wood chips are conventionally treated in batch digestion today with black liquor before digestion of the chips with white liquor. Grounds for this are for example the advantages provided by the black liquor treatment, for example an improved energy economy. Naturally, this presupposes optimal operation of the process. Removal of soap from the solutions containing soap helps in reaching this goal. The soap content of the black liquor in a batch digestion depends on the spot of the displacement process in the digester from which the liquor is recovered. As described above, black liquor is discharged from the digester during the displacement at least to two tanks. The soap content of the black liquor increases when the wash filtrate introduced into the displacement process from brown stock washers is discharged from the digester. In Scandinavian circumstances, the soap content of pulp entering a brown stock washing department may be 80-100 kg/t.sub.m. By regulating the soap content of the wash filtrate to be brought to the displacement, the soap volume ending up in the digester may be reduced as black liquor containing brown stock wash filtrate is typically used in the impregnation of the next digestion batch. Further, it is advantageous to remove soap from this black liquor before this impregnation. Also the soap content of the black liquor removed at the beginning of the displacement process may be reduced if desirable. All these measures may be used to reduce the soap amount entering the blow tanks and further the brown stock washing department with the pulp.
In continuous digestion, black liquor is recirculated to pretreatment, sludging and impregnation of chips. Black liquor is removed from the digester and, if desired, transferred to flash tanks before it is reused. The potential problems caused by soap may be avoided by controlling after the flash tanks the soap content of the black liquor to be used for pretreatment of chips.
When more efficient soap separation is pursued it should be kept in mind that fibers are separated both into black liquor and into wash filtrates which hamper evaporation of black liquor by fouling evaporator surfaces and thus reduce the efficiency of the process and in the worst case even interrupt the production. For this reason, prior art methods tend to separate fiber fraction from black liquor before evaporation thereof by a drum filter in which fiber fraction remains onto a wire surface of a filter drum while black liquor is filtered through a fiber layer and the wire surface and is pumped onward for further treatment, for example evaporation. A process of this kind has been disclosed for example in the PCT application no. WO 9303815. A drawback of these processes is that only a part of the black liquor is guided to recovery of chemicals while a large portion is returned for reuse for example in the impregnation of chips. Thus, it is quite unnecessary to separate fiber fraction from a liquor which in any case is returned to the digestion process.
Just as black liquor, also the soap fraction separated/to be separated from the liquors of a digestion plant and a wash plant contains fibers. Until now, fibers have not been separated from the soap fraction because mill experience has shown that a filter drum used in the filtering of fibers from black liquor is not applicable for this purpose. The composition of this fiber fraction is significantly different from that of black liquor. The soap fraction is a fluffy lumpy material separated from the surface of liquor for example in a separation tank as an overflow. If this fluffy lumpy fraction is distributed onto a drum filter in which a fiber mat is gradually formed onto a wire, it is clear that lumpy soap remains on the same side of the wire surface as the fiber mat and consequently only black liquor becomes separated from the fibers and soap. The separation of fibers from the soap fraction is further hampered by the fact that fibers travelling with the soap fraction are mostly attached to the soap lumps. When this kind of soap fraction is carried without separation of fibers to an evaporation plant the fiber fraction impedes the operation of the evaporator. Further, if the soap is used in the production of tall oil, the fiber fraction hampers the tall oil cooking process.
In U.S. Pat. No. 3,109,839 a process for production of organic acids has been diclosed. The process uses alkali soap skimmings from sulphate pulp manufacture as raw material for organic acids. The process includes treating the soap skimmings with both dispersing agents and mineral acids to decompose alkali metal soaps and to form organic acids and alkali metal and alkaline earth salts. After the decomposition of the soaps the reaction mass is transferred to a screen for separating fibers from the reaction mass. The process avoids the blockage of screen surfaces by means of breaking the soap molecules into smaller molecules which do not interfere with the fiber separation.
An object of the present invention is, among other things, to eliminate the problem mentioned and to provide a method of reducing the fiber content of soap-containing solutions discharged from a pulping process so that fibers do not cause problems in further treatment of the soap fraction or a combined fraction of soap and black liquor.
The characteristic features of the present invention developed for example to reach the object mentioned above are disclosed by the appended patent claims.